1. Field of the Invention
The present invention relates to an endoscope objective lens that has a simple configuration and is suitable for a compact, high-resolution image acquisition device usable in laser treatment and the like, and to an endoscope using the same.
2. Description of Related Art
In endoscopes used in the medical field etc., in order to improve the operability and reduce stress on patients, it has been necessary to reduce the diameter of endoscope insertion portions and to reduce the length of rigid portions at the tip. Therefore, objective lenses installed therein must be configured to have a small outside diameter and a small overall length. Compact endoscope objective lenses having a simple configuration are known (for example, see The Publication of Japanese Patent No. 4245985 and Japanese Unexamined Patent Application, Publication No. 2007-249189).
On the other hand, in the case of endoscopes usable in laser treatment and the like, a filter, such as a laser-light cut filter or a color-correcting filter, needs to be inserted into the objective lens. Endoscopes having such filters are known (for example, see the Publication of Japanese Patent No. 4245985, the Publication of Japanese Patent No. 4229754, the Publication of Japanese Patent No. 3574484 and Japanese Unexamined Patent Application, Publication No. 2004-354888). In the Publication of Japanese Patent No. 4245985, the Publication of Japanese Patent No. 4229754, and the Publication of Japanese Patent No. 3574484, a filter is disposed near the image plane, not at a position immediately behind the aperture stop where the angle of incidence on the filter is large. In Example 9 in Japanese Unexamined Patent Application, Publication No. 2004-354888, a filter is disposed in immediate proximity to the image plane. In Example 11 in Japanese Unexamined Patent Application, Publication No. 2004-354888, filters are disposed immediately in front of the aperture stop and between the aperture stop and a combined lens disposed therebehind.
Meanwhile, in order to improve the diagnostic capability using an endoscope, it is important to improve image quality by correcting various optical aberrations. Endoscope objective lenses achieve a wide angle of view by employing a retrofocus structure, in which a lens group having negative refractive power is disposed on the object side of the aperture stop, and a lens group having positive refractive power is disposed on the image side of the aperture stop. However, because this configuration is asymmetrical with respect to the aperture stop, correcting lateral chromatic aberration is especially difficult. Objective lenses in which such lateral chromatic aberration is effectively corrected are known (for example, see Japanese Unexamined Patent Application, Publication No. 2007-249189).